The statements in this section merely provide background information related to the present disclosure. Accordingly, such statements are not intended to constitute an admission of prior art.
Known electric machines include permanent magnet electric machines. One embodiment of a permanent magnet electric machine includes a multiphase interior permanent magnet (IPM) electric machine having an annular stator into which a rotor is inserted. The stator includes an annular stator core and a plurality of electrical windings. The stator core includes a plurality of radial inwardly projecting teeth that are parallel to a longitudinal axis of the electric machine and define an inner circumference of the stator. Contiguous radial inwardly projecting teeth form radially-oriented slots. The electrical windings are fabricated from strands of suitable conductive material, e.g., copper or aluminum, and are woven or otherwise arranged into coil groups that are inserted into the radially-oriented slots between the teeth. The electrical windings are arranged electrically in series in circular fashion around the circumference of the stator core, with each electrical winding associated with a single phase of the electric machine. Each coil group of the electrical windings provides a single pole of a single phase of machine operation. The quantity of radially-oriented slots in the stator core is determined based upon the quantity of phases and poles of the electrical wiring windings for the electric machine. Thus, a three phase, two-pole machine has electrical windings that are configured as six coil groups, with the coil groups configured in six slots or a quantity of slots that is a multiple of six. Current flow through the electrical windings is used to generate rotating magnetic fields that act on a rotor to induce torque on a shaft of the rotor.
Known rotors for permanent magnet electrical machines include a rotor core attached to a rotating shaft that defines an axis of rotation. Known rotors have a plurality of rotor magnets positioned around the circumference near an outer surface of the rotor core, with each rotor magnet aligned longitudinally with the axis of rotation.
An air gap between teeth of a stator and an outer surface of the rotor is a design feature of an electric machine and is necessary to accommodate manufacturing tolerances, facilitate assembly, and address other known factors. An air gap is preferably minimized, as an increased air gap correlates to reduced magnetic flux and associated reduced output torque.
When electric current flows through stator windings, a magnetic field is induced and acts upon the rotor magnets to induce torque on the rotor shaft. When the magnetic field induces sufficient torque to overcome bearing friction and any induced torque load on the shaft, the rotor rotates the shaft.
In operation, discontinuities in machine torque output including torque ripples are associated with magnitude of the air gap. The air gap and the associated discontinuities in the machine torque output affect maximum machine torque output and affect noise, vibration, and harshness performance of the electric machine.
Known design factors for permanent magnet electric machines include factors related to magnetics, mechanics, thermodynamics, electronics, acoustics, and material sciences. Performance requirements, packaging constraints and costs impose constraints that affect design features. Performance requirements include maximum machine torque output and maximum rotational speed, torque ripple, and cogging torque. The torque ripple and cogging torque affect noise, vibration, and harshness performance of the electric machine. Known permanent magnet electric machines have flux distribution due to the permanent magnets and the armature magneto-motive forces that are non-sinusoidal with respect to the angular rotor position. A non-sinusoidal flux distribution introduces torque pulsations that are reflected as speed ripple, noise and vibration. Torque pulsations may degrade performance of a permanent magnet electric machine and are preferably minimized Torque pulsations affect performance, including efficiency, audible noise, vibration, and harshness. Effects upon performance vary at different operating points, i.e., torque pulsations may vary in response to operating at different speeds and torque outputs. Known strategies to reduce or minimize torque pulsations include skewing locations of magnets in a rotor to minimize torque ripple, adjusting specific design features of a stator and/or a rotor to achieve a minimum torque ripple or achieve a maximum machine torque output at a single operating point, and executing control strategies to generate an inverse torque component through the stator current.